Power efficiency and linearity in the transmitting function of a transmitter which can control its output variably are evaluated as indices for measuring the performance of the apparatus. The power efficiency and linearity in the transmitting function are most important indices for expressing the performance of the apparatus particularly in a high-frequency modulation transmitting apparatus such as a mobile phone.
A so-called class A operation amplifier is widely used as an amplifier provided in a last stage of such a high-frequency modulation transmitting apparatus. The class A amplifier is low in distortion, that is, excellent in linearity but poor in power efficiency because the class A amplifier always spends electric power resulting from a DC bias component.
Therefore, a method using a region of saturation of input-output power characteristic of a transistor to change a drain or collector voltage (supply voltage) in accordance with an amplitude component of a baseband signal to thereby amplify power has been devised as a method for operating a power amplifier efficiently. For example, to change average output power, the supply voltage is changed in proportion to desired average output power (e.g. see Patent Document 1).
FIG. 9 is a graph showing an example of input-output power characteristic (Pin-Pout characteristic) of the power amplifier.
As shown in FIG. 9, at each VCC, output power Pout increases linearly as input power Pin increases. When the input power is not lower than a certain value, the output power is saturated at a value Ps corresponding to the supply voltage. The region where the output power Pout increases linearly as the input power Pin increases is now called “linear region of the power amplifier”. The region where the output power Pout is saturated regardless of the input power Pin is now called “saturation region of the power amplifier”. Because the output power in the saturation operating region of the power amplifier is decided by the supply voltage as described above, the output power can be changed every moment so that average output power can be changed when the supply voltage proportional to desired average output power (dB unit) is given.
In the example of the power amplifier shown in FIG. 9, while the input power changes from Pi81 to Pi82, the output power changes from Po82 to Po83 when the supply voltage is changed from V82 to V83, and the output power changes from Po84 to Po85 when the supply voltage is changed from V84 to V85. When the supply voltage VCC proportional to desired average output power is given in this manner, the average output power of the amplifier can be changed.
Incidentally, the mobile phone network has come into the third generation standard system era in recent years. A mobile phone method such as a W-CDMA mobile communication method etc. has been used for mobile multimedia service. In such a third generation mobile phone method, the output level range required of the power amplifier is widened compared with the conventional one. (Patent Document 1) Japanese Patent No. 3,044,057